Field of the Invention
The present invention relates to x-ray and gamma-ray generation and more particularly to x-ray and gamma-ray generation via laser Compton scattering.
Description of Related Art
Laser Compton scattering (sometimes also referred to as inverse Compton scattering) is the process in which an energetic laser pulse is scattered off of a short duration, bunch of relativistic electrons. This process has been recognized as a convenient method for production of short duration bursts of quasi-monoenergetic, x-ray and gamma-ray radiation. In the technique, the incident laser light induces a transverse dipole motion of the electron bunch which when observed in the rest frame of the laboratory appears to be a forwardly directed, Doppler upshifted beam of radiation. The spectrum of any laser Compton source extends from DC to 4gamma squared times the energy of the incident laser photons for head on laser-electron collisions. (Gamma is the normalized energy of the electron beam, i.e., gamma=1 when electron energy=511 keV.)
By changing the energy of the electron bunch, beams of high energy radiation ranging from 10 keV x-rays to 20 MeV gamma-rays have been produced and used for a wide range of applications. The spectrum of the radiated Compton light is highly angle-correlated about the propagation direction of the electron beam with highest energy photons emitted only in the forward direction. With an appropriately designed aperture placed in the path of the x-ray or gamma-ray beam, one may create quasi-monoenergetic x-ray or gamma-ray pulses of light whose bandwidth (DE/E) is typically 10% or less. The present inventor has been particularly interested in the generation of narrow bandwidth (bandwidth of the order 0.1%) gamma-rays that may be used to excite isotope-specific nuclear resonances. Such beams of gamma-rays may be produced through optimized design of interaction of the laser and electron and with the use of high-quality laser and electron beams whose respective spectra are less than 0.1%.
One fundamental limitation of the laser Compton sources is the small cross section for laser and electron interactions. This cross section known as the Thomson cross section has a magnitude of only 6E-25 cm2. The inverse of the Thomson cross section represents the number of photons required per unit area to achieve unity probability of scattering. For any appreciable probability of interaction, one requires both high photon and electron densities. Typically this is achieved by focusing both the electron and the laser pulse into the same small volume in space and time.
Referring now to the drawings, FIG. 1 illustrates the classical geometry for laser Compton scattering where a single high charge, electron bunch 10 interacts with a single, high energy, laser pulse 12, both of approximately the same short time duration and both of approximately the same transverse size at the point of interaction. Note that the electron beam retains its minimum spot size over a greater distance than the laser pulse for the same minimum spot size. The figure illustrates the electron beam envelope 14, the laser beam envelope 16, the confocal region 18 of the laser focus, and Compton output light 20.
The laser pulse energy required to achieve unity efficiency (one scattered x-ray or gamma-ray per electron) scales as the square of the laser spot diameter. Smaller spots require less laser energy to create the same number of photons from the same charge electron bunch. Because the range over which the laser retains its smallest spot size (confocal parameter) scales as square of the spot size, the maximum duration of the laser pulse for which effective overlap with the electron bunch occurs also decreases in proportion to the square of the spot diameter. Because of the relativistic motion of the electron bunch, it is typical that the region over which the electron bunch retains its smallest transverse extent is greater than that of the laser pulse if both the electron beam and the laser beam are focused to same spot size. For diffraction-limited, green laser light, and practical spot sizes of order 10 microns radius, the required laser energy for 100% scattering efficiency (i.e., one scattered photon for each electron in the electron bunch) is ˜1.8 J while the transit time of the laser pulse through the focal region is of order 5 ps. Typical narrow bandwidth systems operate with 1% to 10% scattering efficiency in order to avoid nonlinear broadening effects.
The time averaged output from laser Compton sources can be increased by increasing the number of electron bunches per unit time produced by the accelerator. In modern, room temperature accelerator systems it is possible to create a long train of electron bunches (so called micro-bunches) whose temporal spacing can be as small as the period of the RF frequency driving the accelerator. The maximum number of bunches in the micro-bunch train is set by the duration of the RF drive pulse for the accelerator and can be of order 1000. By reducing the charge in each micro-bunch, one may dramatically improve the quality of the electron bunch, i.e., its emittance, energy spread, focusability, etc., and thus improve the quality (bandwidth) of the Compton source. Multi-bunch operation can in principle create a higher flux x-ray or gamma-ray output if sufficient laser photons are available for interaction with all the electrons of the micro-bunch train.
One objective of co-pending U.S. application Ser. No. 13/552,610 titled “High Flux, Narrow Bandwidth Compton Light Sources Via Extended Laser-Electron Interactions,” filed Jul. 19, 2011, incorporated by reference, which is by the same inventor, is to increase the focal spot size of the interaction laser spot to match the unfocused transverse dimension of the electron bunch. In this way, the transit time of the electrons through the laser focus is many RF periods. FIG. 2 illustrates asymmetric mode Compton scattering. The figure shows long pulse 30 interacting with many closely spaced electron bunches 32 as they traverse the interaction region. Notice also the shape of the laser pulse envelope 34 and the electron bunch envelope 36. Compton light output 38 is also shown.
To first order, the interaction of the laser with the electron bunch does not perturb the energy of the laser pulse and each electron bunch sees the same laser field. Many electron bunches will interact with the same laser pulse. This method reduces bandwidth broadening effects due focusing of the laser and electron bunch, simplifies the interaction geometry since the electron beam does not need to be focused and greatly reduces the timing synchronization requirements between the long duration (nanoseconds) laser and the picosecond time-scale electron bunch. On the other hand increasing the laser spot size in the interaction region dramatically increases the laser energy required to produce the same number of x-ray or gamma-rays from a given electron bunch charge in proportion to the square of the spot size. This method also really only becomes practical for the highest frequency RF accelerators where the micro-bunch spacing is minimized, e.g., x-band (1.2 GHz or 83 ps bunch spacing), which is 4× the frequency of s-band (3 GHz or 333 ps bunch spacing) is much better suited to this geometry. In real use, the method is also limited by the ability to safely create large laser foci within the spatial constraints imposed by the accelerator, specifically by damage on the turning optics that direct the laser light into the interaction region.